Quantum dots are three-dimensional semiconductor structures that confine electrons and holes in 3-dimensions and thereby produce energy quantization. Quantum dots are so small that quantum mechanical effects come into play in controlling their behavior. The “dots,” which up close actually appear to be more pyramidal in shape, have base dimensions on the order of about 200 Å and height dimensions of about 80 Å.
Some years ago, the scientific community realized that one could make a new type of semiconductor laser by using quantum dots in the active layer. Those new lasers, referred to as quantum dot lasers, held out the promise of great benefits. For example, it was expected that the quantum dot lasers would exhibit less-temperature dependent performance, reduced threshold currents, and more efficient operation than existing semiconductor lasers.
Quantum dot lasers work like other semiconductor lasers. Like the semiconductor laser, the goal of a quantum dot laser is to excite the material into a high energy state and then induce it back into its low energy state resulting in a net release of energy, which emerges as a photon.
One technique for fabricating quantum dot lasers involved forming a layer of quantum dots during Molecular Beam Epitaxy (MBE) growth by a self-assembly method known as the Stranski-Krastanov process. Initial layers are grown lattice matched (or coherently strained) to a substrate material. Following the deposition of the active region, a quantum dot layer is then deposited and quantum dots (e.g. InAs) are formed. Completing the laser structure involves depositing further material layers that are lattice matched to the substrate. Other than the quantum dot layer, the preceding and subsequent material layers are really no different than existing semiconductor structures. However, it is the thin layer of quantum dots that holds out the promise of a new level of semiconductor laser performance.
For some people within the community of researchers of quantum dot lasers, it has been the view that being able to establish highly dense and highly uniform quantum dots was essential for achieving the benefits that were predicted for quantum dot lasers. So, part of the research community has put effort into improving the uniformity of the quantum dot layers.